With the development of modern electronics, there has been a parallel development of both method and apparatus suitable for accurately determining electronic device and material characteristics. For example, sampling oscilloscopes have been developed to the point where they can measure device response times of approximately 40 picoseconds.
However, the speed of solid state electronic devices has greatly increased in recent years and has now reached the stage where the conventional approaches, such as sampling oscilloscopes, are unable to accurately determine their response times. For example, gallium arsenide and silicon field effect transistors (FETs) with effective channel lengths of approximately 1 micron and less typically have propagation delays that are estimated to be 30 picoseconds and cannot be measured by sampling oscilloscopes. Techniques have, however, been developed which permit indirect characterization of some aspects of device operation. However, these techniques do not permit evaluation, even indirectly, of all aspects of device operation. Presently, the standard technique used to evaluate high speed electronic devices involves the construction of a ring oscillator having a number of identical devices connected in tandem. The period of oscillation divided by the number of devices then gives an estimate of the propagation delay per device. This technique provides a number that is, at best, an average for the comparison of different devices, and fails to identify the specific factors which influence device response time and which must be understood if improved devices are to be designed.
It would, of course, ideally be preferable to make a single direct measurement of the impulse response of a single device. Although the sampling oscilloscopes previously mentioned may be constructed with rise times as short as 25 picoseconds, pulse generators with similar speeds are not available. Furthermore, the problem of synchronizing the pulse generation and sampling functions limits the use of sampling oscilloscopes to time scales greater than approximately 50 picoseconds.
Recently, sampling systems having high resolution, less than 10 picoseconds, and capable of use with individual devices have been developed. See, for example, Applied Physics Letters, 36, pp. 1005-1007 and pp. 1008-1010, both dated June 15, 1980. These articles describe apparatus for measuring ultrashort current pulses with Josephson devices. While these sampling systems permit measurement of pulses having rise times as short as 10 picoseconds, they suffer the drawback of using superconducting devices which require immersion in liquid helium.